Fully Constraint Platform in Deepwater

ABSTRACT

A method and apparatus for securing a buoyant, surface structure in deepwater, such as those used in offshore oil and gas drilling and producing, is disclosed. The surface structure is water surface piercing or fully submerged. Buoyancy tanks inside the surface structure are used as liquid vibration dampers to enhance the motion performance. All tethers are sufficiently taut and some or all of them are inclined. The attachment points of the tethers on the surface structure are at same or different elevations. An alternative attachment method, named as “single tether-multiple attachment leg”, is also disclosed. The tether system provides stiffness in both vertical and horizontal direction so that all six degrees-of-freedom motions of the structure, namely, surge, sway, heave, roll, pitch, and yaw are constrained with stiffness. This fully constraint platform (FCP) has minimum motion in winds, currents, swells, and surface and internal waves, and therefore has many practical uses, as outlined herein.

BACKGROUND OF THE INVENTION

A platform in the ocean, such as those used in offshore oil and gasproduction, will respond to winds, waves, and currents. Its motion ischaracterized by six-degrees of freedom (abbreviated as DOFs): three aretranslational (surge, sway and heave), and three are rotational (roll,pitch, and yaw). The environmental loads can force the platform to movein one or more DOFs.

A design should ensure minimum motion in these DOFs such that thefunctional requirements of the platform can be met. For example, if theplatform is used for drilling a well through the water column, it shouldnot have large horizontal or vertical motion. Otherwise, the drillingoperation cannot be conducted. The motion of a platform can beclassified as static or dynamic. One example of the former is that dueto steady drag loads from ocean currents. The motion due to surfaceocean waves is dynamic. In many cases, there are both static and dynamicmotions.

When the water depth is shallow, e.g., on the order of 500 ft or less,the platform can be designed such that sufficient structural stiffnessis provided (by steel or steel reinforced concrete) to all six DOFs. Asa result, the platform will have minimum motion. This type of structuresis termed as a “fixed” platform. Many platforms in shallow water of theGulf of Mexico, for example, belong to this category.

When the water depth is greater, e.g., over 1,000 ft, much more steel orconcrete will be needed to have similar motion characteristics if thesame fixed platform design concept is adopted. This is because that thehorizontal stiffness of the support structure is inversely in proportionto the cubic of the water depth. The concept of “fixed platform” will nolonger be deemed economical in deepwater: the amount of steel orconcrete required to provide such stiffness to the platform is simplyphenomenal.

New platform concepts for deepwater, such as tension-leg platforms,abbreviated as TLPs (such as that disclosed in U.S. Pat. No. 3,577,946),semi-submersibles, and spar platforms (such as that disclosed in U.S.Pat. No. 4,702,321), therefore emerge.

TLPs are now widely used in deepwater. To date, there are overtwenty-five installations in world's ocean. Referring now to FIG. 1 (a),a TLP consists of: (1) a buoyant, surface structure 203 which supportsdeck(s) and structures and equipment on the deck(s), (2) a tether system202, and (3) a foundation structure 201 which is secured on theseafloor. The tethers are usually tubular in shape and are made fromhigh strength steel.

The working principle of a TLP is as follows. The buoyancy generated bythe buoyant, surface structure is larger than the combined weight of itsown and of everything it carries. The excessive lift force is taken bythe tether system, which is then sustained by the foundation. Thetension in each tether is often on the order of a few thousandskilo-pounds (kips). A TLP can support a large amount of payload, such asthe weights of a drilling rig, deck(s), equipment (for initiallyprocessing oil and gas) on the deck(s), and man quarters.

The TLP concept uses a tether system to constrain a surface structurevertically. The vertical stiffness of a TLP depends on the Young'smodulus of the tether material, the cross section area of the tethers,or the amount of steel, and the length of the tethers. Let's say thetether system consists of five steel tubes, 25-in. in outside diameterand 1-in. wall (the cross-sectional area is therefore 75.4 in²), tosupport a surface structure in 1,500 ft of water (assuming the length ofthe tether is 1,500 ft). The vertical stiffness of the tethers would be5×75.4 in²×30,000 ksi/1,500 ft=7,540 kips / ft.

The horizontal stiffness of a TLP is controlled by the tension intethers and the length of the tether. For example, if the tension intethers is 6,000 kips and the length of the tether is 1,500 ft, thestiffness would be 6,000 kips/1,500 ft=4 kips/ft (this is the stiffnessat a zero offset. When there is an offset due to environmental loads,this stiffness will change nonlinearly). This means that to move thesurface structure horizontally by one foot, a force of 4,000 pound isneeded. For a surface structure of typically one to two hundred feet insize and often over 20,000 kips in weight (of its own and those itcarries), this is really a very small force. In other words, thehorizontal stiffness of a TLP is very small, only 1/1,885 of thevertical stiffness, for the above example. Clearly, the stiffnessgenerated from the axial stretch of the tether is significantly higherthan that from the tension.

Because of the difference in stiffness by such a large magnitude, a TLPwill behave very differently in its vertical and in its horizontaldirection. This platform concept has minimum heave, pitch, and rollmotion, but its surge, sway, and yaw can be large, if there is a largeforce (either static or dynamic) to excite its motion in any of theseDOFs. Referring now to FIG. 1 (b), e.g., the surface structure 203 willoffset under environmental load 204.

In high winds, large waves, and strong currents, such as those generatedby a 100-year storm or a 100-year ocean current or an internal wave(such as that encountered in Southeast Asia), a TLP could have anexcursion on the order of hundreds of feet. This type of motioncharacteristic is

-   (a) a challenge to the design and operation of the facilities and    structures on a TLP, such as the deck, deck equipment, and risers    and flowlines,-   (b) a negativity to human's health for those working on the    platform, and-   (c) unacceptable if the TLP concept is intended for other uses, such    as those disclosed in this invention.

The present invention solves the large horizontal motion problem of aTLP.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for securing aplatform in deepwater. In another aspect, the present invention relatesto methods and apparatus for fully constraining a platform in deepwater.

SUMMARY OF THE INVENTION

The present invention provides a fully constraint platform for use inworld's ocean. The mechanism of this system is different from otherdeepwater platforms, such as the TLPs, in that all 6 degrees-of-freedom(abbreviated as DOFs) of the buoyant, surface structure, namely, surge,sway, heave, roll, pitch, and yaw, are constrained with the axialstiffness of the tethers. The tethers are taut and some or all of themare inclined. The attachment points of the tethers on the buoyantstructure are at different vertical heights. The inclination can be madesuch that the horizontal distance is as large as the depth of the watercolumn. This fully constraint platform (FCP), supporting one or moredecks, has minimum motions in winds, currents, swells, and surface andinternal waves.

Because of its motion characteristic and its capability to carry asignificant amount of weight, this platform concept can find a number ofapplications in the deepwater regions of world's ocean. Morespecifically, the buoyant, surface structure can be used as:

-   (a) a stand alone offshore drilling platform with a tender vessel    assisting, or-   (b) an offshore production platform with or without drilling and/or    well servicing (work-over) capability, or-   (c) an anchored platform in product offloading and other offshore    operations, which involves more than one surface structures adjacent    to one another, or-   (d) an offshore work platform, or-   (e) an offshore platform to support a wind or current turbine or a    transmission line tower or a lighthouse, or-   (f) an artificial island or a group of artificial islands, or-   (g) a marine terminal or a floating harbor, or-   (h) an ocean airport with multiple runways, or-   (i) a host platform for a nuclear reactor.

These and other aspects of the invention will become apparent to thoseof skill in the art upon review of this specification, including itsdrawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tension leg platform (TLP) and its offset underloads

FIG. 2 illustrates a fully constraint platform (FCP)

FIG. 3 illustrates offset due to current loads

FIG. 4 illustrates surge due to wave loads

FIG. 5 illustrates a minimum FCP

FIG. 6 illustrates attachment points of tethers at different elevations

FIG. 7 illustrates a tether separation angle table

FIG. 8. illustrates a method to distribute tether attachment pointsvertically

FIG. 9. illustrates various configurations of single tether-multipleattachment leg (STMAL) system

FIG. 10. illustrates a minimum FCP with a STMAL system

FIG. 11. illustrates a fully submerged FCP

DETAILED DESCRIPTION OF THE INVENTION

The present invention, termed as a fully constraint platform andabbreviated as “FCP”, is derived from the TLP concept. It solves thelarge horizontal motion problem of a TLP. Referring now to FIG. 2, a FCPrequires the redistribution of the axial stiffness of the tethers 302,anchored at seafloor 301, by making some or all of them inclined at anangle from the vertical. The inclination can be such that the horizontaldistance is as large as the depth of the water column. This way, thestiffness in both the vertical and horizontal direction (in particular)will be dependent upon the Young's modulus of the tether material, thecross-sectional area of the tether, and the length of the tether, ratherthan the tension in the tether as in the case of a TLP. Since the motionof the buoyant, surface structure 303 in any of the six degrees offreedom (abbreviated as DOFs) will stretch the tether and thus will beresisted by its axial stiffness, the buoyant, surface structure is fullyconstrained. Furthermore, a TLP can be viewed as a special case of theFCP concept.

In the ocean, there are many disturbances which could offset a buoyant,surface structure, such as the winds, currents, swells, surface andinternal waves. Wind, current, swell, and internal wave forces aremainly static or quasi-static in nature (meaning changing at a lowrate), while the surface waves are oscillatory. When a structure issubjected to steady loads, its motion is controlled by its stiffness.The greater the stiffness a structure has, the smaller the motion itexperiences. Therefore, for a properly sized FCP, the steady motion canbe minimum (as will be illustrated later in this section with anumerical example). In contrast, a TLP will have large horizontal offset(due to its low horizontal stiffness).

Raising the stiffness in a system will change its dynamiccharacteristic, to such a degree that the motion, in each of the sixDOFs, is controlled by the stiffness. This is one of the key differencesin methodology between the present invention and the priori art. A FCPsystem can be designed such that the natural frequencies of all sixdegrees of freedom will be above the surface wave frequencies ofsignificant energy, resulting in motion characteristics similar to thoseof a fixed platform.

Furthermore, since some or all the tethers of a FCP are inclined,vortex-induced vibration or “VIV” is less likely to occur (compared tovertical tethers), due to smaller normal incident flow speeds. Themotion of a TLP is strongly dependent on its payload. If the payload isincreased, the tether tension will decrease, which will affect itsstiffness and therefore its ability to control the motion in thehorizontal direction in particular. For a FCP, as long as the payloaddoes not exceed the buoyancy, its stiffness will remain unchanged.

To further illustrate the merit of the FCP concept, a numerical examplefrom a computer model is provided herein, which compares the offset of aFCP and of a TLP in currents in FIG. 3, and in waves in FIG. 4. In FIG.3, the horizontal axis is the current speeds in knots (nautical milesper hour), the vertical axis is the platform offset in feet. The offsetof a TLP is indicated with “diamond”, and that of a FCP with “square”.In FIG. 4, the horizontal axis is the surface wave height in feet, thevertical axis is the surge motion amplitude (dynamic) in feet. The surgeof a TLP is indicated with “diamond”, and that of FCP with “square”. Inboth cases, the motion of a FCP is significantly less than that of aTLP.

The key of the invention is therefore:

Some or all of the tethers are inclined. In the horizontal plane thetethers can be conveniently distributed that the motion is asomni-directional as possible. The attachment points on the surfacestructure can be at different elevations. This way, any one of the sixDOFs motion will stretch at least one tether, such that its axialstiffness, EA/L, will function. This motion will therefore beconstrained by the axial stiffness of the tethers. The tethers havefinite axial buckling resistance, but large tensile capacity.

To further enhance the dynamic performance of a FCP in ocean waves, thebuoyant, surface structure is partitioned into internal buoyancy (alsocalled ballast) tanks which can be used as liquid vibration dampers(abbreviated as LVDs). The size of the tanks and the level of the fluids(water can conveniently be used) are so determined that a certainexcitation frequency range in the surface wave force is targeted. Whenthere is wave energy around this frequency range, part of the inputenergy will be absorbed via the sloshing of the liquids inside theballast tanks (Lamb, 1993). This feature of the invention isparticularly useful since the natural frequencies of the heave, pitchand roll motion could enter the significant wave energy zone, if the FCPis intended for very deep water. There is a trade off between using theLVDs and more tethers. A threaded-bolt and nut mechanism can also beused to spool and tighten the tethers when needed.

One embodiment of a FCP (FIG. 5) is a minimum structure. It consists ofa buoyant, surface structure 403 (water surface piercing) and threetethers 402, each at an angle from vertical, and 120-degree apart (inplan view). This structure can be used as a working platform or tosupport an offshore wind or current turbine, or a transmission linetower. Three weight or pile anchors 401 can be used to secure thetethers. This structure is intended for use in unmanned cases.

Another embodiment of a FCP (FIG. 6) is that the attachment points ofthe tethers 502, anchored at seafloor 503, are at different elevationsto enhance the rotational rigidity of the surface structure 501, inorder to resist its pitch, roll, and yaw motion. In the horizontalplane, the tethers are uniformly spread. This way, more planes ofsymmetry is achieved so that the motion can be more omni-directional.The method to spread the tethers uniformly is to employ the followingformula:

a=360/n

where a is the separation angle between two adjacent tethers in thehorizontal plane and n is the number of tethers. Note that theattachment point of each tether can be at different elevation. Thenumber of levels, the number of tethers at each level, the total numberof tethers, and the corresponding separation angles are listed in FIG.7. The tethers can also be grouped together to attach to each corner ofa buoyant, surface structure with distinct corners.

The method to determine the vertical elevation of each level of tethersis the following:

-   -   (a) find the center point of the external force (e.g., if the        draft of the surface structure is 100 ft and the current is        uniform, the center of the drag force is at 50 ft from the keel)    -   (b) distribute the level of tethers uniformly about this center        point (as is illustrated in FIG. 8).        Each tether can have multiple attachment legs (single        tether-multiple attachment leg or abbreviated as STMAL), in        order to:

-   (a) reduce costly multiple tethers and foundation anchors on the    seafloor, and

-   (b) increase the motion stability of the surface structure for the    same number of tethers.

A number of configurations of this STMAL system are illustrated in FIG.9 The pivot point (from which the legs are coming out) is indicated by adouble circle. This pivot point can be inside or outside of the water.The length of each leg can vary to accommodate the distance from thepivot to the surface structure. Furthermore, this configuration cancascade. An embodiment to further illustrate this method is shown inFIG. 10, where the buoyant, surface structure 601 is constrained by theSTMAL system 602, which is secured on the seafloor via anchors 603.

Another embodiment of a FCP is illustrated in FIG. 11, where thebuoyant, surface structure 703 is completely submerged. In this case, atruss structure 702 is placed on top of the surface structure 703 tosupport deck 701 or other functional structures. The buoyant, surfacestructure 703 is constrained by the tether system to 704, which issecured on the seafloor via anchors 705.

1. A fully constraint platform in deepwater comprising: (a) a buoyant,surface structure, consisting of a single structure or a plurality ofsuch structures in various orientations each of which is a hollowcolumn, with the to center of the formed buoyant, surface structureeither solid or hollow to serve as a moon pool, and with deck(s) placedon top of the buoyant, surface structure. (b) a tether system,consisting of a plurality of tethers, to attach to the said surfacestructure, each of which is sufficiently taut and has an angle ofinclination from 0 degree to approximately 45 degrees from vertical. (c)a foundation system with anchors to secure each of the tethers,comprising driven piles or suction piles or gravity based weights. 2.The system of claim 1(a), further being water surface piercing orcompletely submerged, wherein in the latter case, a truss structure isplaced on top of the buoyant, surface structure, to support deck(s)along with other functional structures and equipment.
 3. The system ofclaim 1(a), wherein each single structure is further partitioned intoone or more buoyancy tanks filled with fluids to function as liquidvibration dampers and as means to balance the tension in the tethers. 4.The system of claim 1(b), wherein the attachment points of tethers tothe buoyant, surface structure are at one or more elevations.
 5. Thesystem of claim 1(b), wherein some or all of the attachment points areeither outside of or inside of the water.
 6. The system of claim 1(b),wherein each attachment point to the surface structure is a steel block(a porch) which is separated from and welded to the main structure. 7.The system of claim 1(b), wherein the tethers are made from carbon fiberor polyester or steel.
 8. The system of claim 1(b), wherein the tethersare a chain or a rope or a tube or a combination of segments made from achain or a rope or a tube.
 9. The system of claim 1(b), wherein when thetethers are entirely tubular, is also a conduit to transfer fluids orcontains power, fluid and communication cables.
 10. The system of claim1(b), wherein one or more tethers have single or multiple attachmentlegs attached to the buoyant, surface structure, which can furthercascade.
 11. The system of claim 1(b), wherein further accompanying amethod to uniformly distribute tethers in the horizontal plane, and amethod to distribute the tethers in the vertical plane, as described inFIG. 8 and FIG. 9, respectively.
 12. The system of claim 1(b), whereinfor the system of 1(a) with distinct corners, the tethers are dividedevenly into a number of small tether groups, with the number of groupsmatching the number of corners of the said structure.
 13. The system ofclaim 1 is used as: (a) a stand alone offshore drilling platform with atender vessel assisting, or (b) an offshore production platform with orwithout drilling and/or well servicing (work-over) capability, or (c) ananchored platform in product offloading and other offshore operations,which involves more than one structures adjacent to one another, or (d)an offshore work platform, or (e) an offshore platform to support a windor current turbine or a transmission line tower or a lighthouse, or (f)an artificial island or a group of artificial islands, or (g) a marineterminal or a floating harbor, or (h) an ocean airport with multiplerunways, or (i) a host platform for a nuclear reactor.